The present invention relates to the measurement and display of video signals, and more particularly to a sync vector generator for composite vectorscopes for measuring the horizontal sync jitter of a video signal in degrees of a subcarrier cycle.
For television video signals it is often desired to measure the horizontal sync jitter of the video signal, as well as the phase relationship between horizontal sync and subcarrier color burst (SC/H phase measurement). To measure horizontal sync jitter on a conventional oscilloscope/waveform monitor to a desired accuracy of less than one nanosecond is difficult since the sync triggering in a conventional oscilloscope is not optimized for triggering on the sync of a video signal. Some of the newer oscilloscopes have specialized video triggers, but the jitter and average picture level (APL) sensitivity are not good enough to evaluate sync timing in the few nanoseconds range. In addition most oscilloscopes do not have sweeps faster than 10 nsec/div whereas a vectorscope, which is specially designed to display television signals, can resolve one nanosecond subcarrier timing, approximately 1.5 degrees, easily. Also it is sometimes convenient to measure the jitter in terms of subcarrier degrees since television genlock systems occasionally must lock a subcarrier related oscillator, or subcarrier synchronous sample clock, to a television horizontal sync source. In this case, as well as others, it is desirable to quantify the jitter in terms of subcarrier phase in degrees to allow mapping directly into chroma hue errors of a composite video signal, chroma being conveniently expressed in terms of degrees.
Many digital video signal processors, either component or composite, have sync locked clocks and it is often necessary to evaluate the analog sync output jitter with respect to a house composite reference, as disclosed in co-pending U.S. Pat. No. 5,010,403 issued Apr. 23, 1991 to Edward D. Wardzala entitled "Measurement of Timebase Jitter for Component Video." This is accomplished with a sync vector display when the vectorscope is externally referenced. The input signals may be component, i.e., without a color burst component, and the sync jitter can be evaluated directly in subcarrier degrees.
For SC/H phase measurement conventional SC/H phase indicators average a large number of sync edges together, the sync edge being defined as the fifty percent (50%) point on the leading edge of the horizontal sync pulse, to display some average SC/H phase. This masks or at least reduces any large SC/H phase error that occurs sporadically for only a few or even a single line.
Therefore what is desired is a measurement apparatus that provides a measure of horizontal sync jitter in degrees of the subcarrier frequency as well as line by line SC/H phase measurements.